Sean Mckeever says:
"After that, there's something pretty big. There's a character that I wanted to put on the Teen Titans from the beginning, but I was not allowed to do it when I first started. But now it seems that I'm allowed to."
Ok we know it ain't Supergirl, so what other hero could it be?
My guess is Freddy Freeman, because he probably couldn't be used elsewhere because of the Trials of Shazam miniseries but since they are finished....